[U-Boot] [PATCH 30/53] binman: Update documentation for image creation

[sjg at google.com]

There are a few more steps in the process now. Update the documentation to
reflect this.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

 tools/binman/README | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

Applied to u-boot-dm, thanks!